Central Asia (CA) is experiencing one of the fastest growing HIV epidemics in the world with an annual doubling of infection rates in some areas since 2000. The mass migration in CA following the collapse of the Soviet Union has been identified as a key factor in fueling the heterosexual spread of HIV and other STIs in the region. The proposed study responds to PA-07-147 which strongly encourages international research on cross-cultural determinants of sexual risk behaviors and incidence of HIV and other sexually transmitted infections (STIS) among underresearched populations of heterosexual men. The proposed four-year study will rigorously examine the influence of theory-driven, multi-level determinants on sexual HIV risk behaviors and incidence of HIV/STIs among male migrant market workers from Barakholka Market in Almaty, Kazakhstan from four dominant countries in Central Asia (CA), including Kazakhstan, Kyrgyzstan, Tajikistan, and Uzbekistan and a comparison group of non-migrant, market workers who are permanent Almaty residents and citizens of Kazakhstan. The proposed study will use mixed methods of a qualitative study of 60 market workers followed by a longitudinal study with a random sample of 2,000 market workers from the Barakholka market in Almaty, Kazakhstan (400 migrant workers from each of the four CA countries and a comparison group of 400 non-migrant market workers). The Barakholka market has approximately 30,000 stalls and is one of the biggest trading centers in CA. The proposed grant application has been informed by two feasibility pilot studies that were conducted with migrant market workers at the Barakholka Market. The proposed study findings will have HIV prevention, intervention and policy implications. Furthermore, the proposed study may serve as a prototype for conducting longitudinal HIV/STI research with migrant workers, which has been identified as a priority area of needed research by WHO, World Bank, UNAIDS and international HIV experts. The study will be led by Dr. Nabila El-Bassel, Louisa Gilbert, Dr. Elwin Wu and Dr. Peter Bearman from Columbia University (CU) and the investigative team from Almaty including Dr. Assel Terlikbayeva, (site Principal Investigator);Dr. Baurzhan Zhusupov (Co-Investigator) from the Centers for Disease Control (CDC) in Almaty and Dr. Gulsum Askarova (Co-Investigator) from the Republican Skin and Veneral Disease Institute. This multidisciplinary collaboration will be strengthened with the expertise of Dr. Christopher Beyrer (Co-Investigator), Director of the John Hopkins Fogarty AIDS International Training and Research Program, Dr. Mary Jane Rotheram-Borus (Consultant) from University of California at Los Angeles and Dr. Naihua Duan, Director, Division of Biostatistics at CU College of Physicians and Surgeons. PUBLIC HEALTH RELEVANCE: Migrant men in Central Asia have been identified as a key bridge population in facilitating the heterosexual spread of HIV/STIs. This grant application examines different social and cultural contexts and determinants that influence sexual risk behaviors that spread HIV and other sexually transmitted infections (STIs) among young migrant market workers in Central Asia. The findings from this study will inform future prevention efforts to stem heterosexual transmission of HIV and STIs in Central Asia among male migrant workers.